Evaluate. Assume $u>0$ when $I$

$u$ appears.

${\displaystyle \underset{\phantom{}}{\overset{\phantom{}}{}}}3t{e}^{-t^2}dt$

${\displaystyle \underset{\phantom{}}{\overset{\phantom{}}{}}}3t{e}^{-t^2}dt=$

$($Type an exact answer.$)$